Additions to Univariate KDEs

I've added 'cdf_eval', 'icdf_eval', 'variance' and 'variance_eval' functions to the UnivariateKDEs. 

I realise that the format of these changes may not be what is wanted in the long run, but I figured starting a pull request would help push discussion along so some choice is made.

ref: https://github.com/statsmodels/statsmodels/issues/904